It is known to implant into a patient a CRT pacemaker with electrodes to stimulate both left and right ventricles of the patient. Such devices typically continuously monitor the patient's heart rhythm and deliver, if necessary, electrical pulses to concurrently stimulate the left and right ventricles to resynchronize them. An interventricular delay (“DVV” or “VVD”) can be applied between the respective moments of stimulation in the two ventricles and adjusted to resynchronize the contraction of both ventricles to optimize the patient's hemodynamic status. One such CRT pacemaker is disclosed, for example, in EP 1108446 A1 and its counterpart U.S. Pat. No. 6,556,866 (assigned to Sorin CRM, previously known as ELA Medical).
Similar to conventional pacemakers, a CRT pacemaker can be rate-responsive including means for controlling the rate of the pacing pulses according to the level of a patient effort. A lower pacing rate is delivered when the patient is at rest, and a gradually increasing pacing rate is delivered as the patient's effort increases. The parameter for determining a patient's effort is measured by an appropriate sensor or combination of sensors, such as a physiological sensor (e.g., a minute ventilation sensor or “MV sensor”) and/or a physical activity sensor (e.g., an accelerometer or “G Sensor”).
The rate-responsive pacing rate can vary between a minimum frequency referred to as a “base frequency” (fbase) and a maximum frequency (fmax) that defines a maximum value for the pacing rate calculated by a rate-responsive algorithm. It should be understood, however, that the instantaneous frequency of a patient's heart rate may be either a spontaneous frequency (e.g., a sinus rhythm) or a stimulated pulse frequency, and that in the latter case, the stimulation pulse frequency is managed by the pacemaker according to the level of patient effort.
EP1059099A1 and its counterpart U.S. Pat. No. 6,622,039 (assigned to Sorin CRM, previously known as ELA Medical) describe a CRT pacemaker providing with a long-term adjustment of the fmax parameter according to the changes in the patient's general condition.
A typical indication for use of a CRT pacemaker is a heart failure condition arising from dilated cardiac cavities together with a significantly reduced ejection fraction and a rapid exhaustion of the patient even in the case of a slight effort being exerted by the patient. For healthy people, during an effort, the heart puts into action two mechanisms to meet increasing hemodynamic needs. First, there is an increase in a heart rate (i.e., spontaneous frequency or sinus rhythm). Second, there is an adaptation of contractility (which affects the ejected volumes). However, in the presence of a heart failure condition, the patient has little adaptability with respect to the contractility of heart muscle.
In addition, patients suffering from heart failure are generally under intensive medical treatment, including administration of a beta-blocker medication to slow their heart rate, and their average spontaneous frequency at rest is generally higher than that of healthy patients. However, these beta-blockers and other drug treatments have an effect of reducing the capacity of the heart rate acceleration. This results in a lower reactivity during an effort.
As a consequence, for patients with class II heart failure as classified by the New York Heart Association (“NYHA”) standards, a rapid exhaustion follows in a moderate patient effort. For patients classified in NYHA classes III and IV, a rapid exhaustion occurs irrespective of the level of patient effort or exercise even for simple activities of everyday life.
It is known that using multisite stimulation for CRT can alleviate some of the consequences of heart failure through better synchronization between the two ventricles. The alleviation is achieved by increasing contractility and increasing the filling phase of the cardiac chambers. However, due to the use of beta-blockers, even in the case of a little patient effort, the responsiveness of a heart rate increase is limited by the negative chronotropic effect of these drug treatments.
Known multisite CRT pacemaker devices may include algorithms to control the rate-responsive pacing. WO 92/03182 A1 describes a conventional single chamber, rate-responsive pacemaker but it does not have CRT functionality. The rate-response in such a device is designed to be as physiological as possible, therefore it provides a slow and progressive variation in the order of several tens of seconds. In one example, such a device provides a transition from the base frequency to the maximum frequency over about sixty seconds.
Such a prior art rate-responsive algorithm, originally intended for a patient suffering from bradycardia, does not produce a desired result for a patient suffering from a cardiac heart failure. Indeed, it may produce an adverse effect. For example, if at the beginning of the patient's effort the patient is in spontaneous rhythm with a frequency greater than the pacing rate calculated by the rate-responsive pacing algorithm, the patient is not stimulated. This induces an additional delay until the algorithm adapts the rate-responsive rate corresponding to the patient's effort level, because the patient only benefits from the rate-responsive feature when a stimulation frequency calculated by the algorithm is higher than the current spontaneous frequency of the patient.
On the other hand, the profile for increasing the pacing rate in the rate-response algorithm is intended to apply a slow and gradual increase in frequency, especially at the beginning of the patient effort. In this case, when the patient begins to be stimulated, the frequency increases slowly for a moderate effort, with a greater acceleration slope only for sustained efforts. Such a frequency rate response profile is, therefore, poorly suited to patients suffering from a heart failure condition because such patients in general rarely perform sustained efforts. As a result, they do not have a significant and rapid increase in their heart rate, and the dynamics of the heart rate increase is limited by the negative chronotropic effect of drug treatments administered to them.